CAMP LEATHERNECK, Afghanistan, July 3 -- Taliban insurgents stepped up attacks Friday against U.S. Marines in southern Afghanistan's Helmand River valley, forcing troops in some areas to spend the day fighting instead of carrying out plans to meet with residents and local leaders.

The stiffest resistance occurred in the district of Garmser, where Taliban fighters holed up in a walled housing compound engaged in an eight-hour gun battle with troops from the 2nd Battalion of the 8th Marine Regiment. The Marines eventually requested a Harrier fighter jet to drop a 500-pound bomb on the compound, which was believed to have killed all fighters inside.

The commanders directing the huge Marine security operation here had said they hoped not to rely on airstrikes, which have resulted in numerous civilian casualties in Afghanistan over the past seven years. Officers here noted with pride Thursday that they had not used bombs or artillery in the first 24 hours of the mission.

But they were left with little choice after the insurgents refused to surrender. Even so, senior officers emphasized that they had watched the building -- from the ground and with surveillance aircraft -- for almost a day before concluding there were no civilians inside.

There was no immediate count of insurgents killed, although ground commanders reported that 30 to 40 were shooting from in and around the compound early in the day. The airstrike also resulted in several secondary explosions, leading Marines at the site to suspect that the house may have contained homemade bombs.

The fight began Thursday, when insurgents attacked the 2nd Battalion's Echo Company, which was conducting a foot patrol after arriving in the area by helicopter. The insurgents subsequently retreated into the housing compound, whereupon an hours-long gunfight ensued.

The firing stopped Thursday evening, after Marine AH-1 Cobra helicopters fired Hellfire missiles into the compound. It resumed Friday morning when a Marine transport helicopter landed to deliver Echo Company pallets of food and water. The helicopter was unscathed, but the fusillade soon escalated to large-caliber machine-gun fire and rocket-propelled grenades.

The incident, which the overall commander of Marine forces in southern Afghanistan called "a hell of a fight," did not result in any U.S. casualties, but it kept Marines in Garmser from meeting with village elders and other residents.

Officers at the combat operations center here noted that their counterinsurgency effort will involve similar offensive actions against insurgents they encounter. But such engagements effectively delay key elements of the Marine stabilization strategy, which is focused on winning the allegiance of the local population by promising protection from the Taliban.

Other Marine units in Garmser were also attacked Friday. But when the Marines returned fire, those insurgents broke contact and retreated. "They're not decisively engaging us," said Lt. Col. Jeff Rule, who supervises the combat operations center at this vast desert base west of the river.

In other parts of Helmand, Marines encountered different obstacles as they sought to introduce themselves to residents. To the north of Garmser, in the district of Nawa, troops have hit several roadside bombs. In the town of Khan Neshin, in the southern part of the river valley, Marines had hoped to meet with town elders, but they did not show. Most people in the town simply stayed indoors, uncertain whether the troops were there to stay and fearful of retribution attacks from the Taliban.

"They're acting cautiously," said Col. Eric Mellenger, the operations officer for the 2nd Marine Expeditionary Brigade, which is commanding all Marine units in Helmand. "They don't want to get caught in the middle."

Although some Marine units have yet to accomplish initial objectives of the mission, including finding homes to rent and convert into troop outposts, Mellenger said the forces have succeeded in disrupting Taliban activities. In many parts of Helmand, Taliban members had moved around freely for years because local police units had fled and the British military, which was responsible for the province until the Marines arrived, did not have enough manpower to patrol every major town along the river valley.

British forces have since focused their efforts on the capital of Helmand, Lashkar Gah, and surrounding towns, where they are mounting a large operation aimed at pressuring the Taliban in conjunction with the Marines. British forces have suffered significant casualties in that mission, including the death of a lieutenant colonel commanding a battalion of the Welsh Guards. He was killed in an explosion Wednesday, British officials said.

Taliban fighters have been "totally destabilized by the number of coalition forces now operating in Helmand," Mellenger said. "They can no longer move around and link up as they want."